The wetting behavior of a liquid on a solid surface is a phenomenon of significant practical importance. The angle of liquid to solid contact on a solid surface is important in diverse areas of science and technology, such as, adhesion, adsorption, lubrication, catalysis, solid-liquid reaction kinetics, heat transfer, electrical conduction, and micro-fluidic devices. This angle of contact, called the contact angle (θ), is one way to measure and assess the phenomenon of liquid-solid wetting.
The contact angle θ of a liquid on a surface may be used to define to what extent, if any, a liquid will “wet” or contact a surface. Whenever a liquid contacts a solid surface, several different types of behavior can be exhibited. At one extreme, a drop of liquid contacting a solid surface will spread out until it forms a thin film on the surface. This is called total wetting and in this case the liquid has a contact angle θ of zero with the surface. At the other extreme, a drop of liquid will sit on the surface like a marble with minimal contact. This behavior is termed total non-wetting and the liquid in this case forms a contact angle θ of 180° with the surface. For situations in between these extremes, a drop will be formed that makes a well-defined contact angle, θ, with the surface. This is called partial wetting.
The standard historical convention applied to the partial wetting behavior is that if the contact angle is less than 90 degrees, the liquid “wets” the surface. If the contact angle is greater than 90 degrees, the liquid “does not wet” the surface and is termed “non-wetting”. In the present document, the terms “wet”, “wetting”, “not wet”, and “non-wetting” will be used to refer to this partial wetting behavior and not to the absolute definitions.
The intrinsic contact angle θi is the angle between a static liquid and a smooth planar horizontal surface. This contact angle is only dependent on the material properties of the liquid and the smooth planar horizontal surface. The apparent or observed contact angle θa will differ from the intrinsic contact angle due to contamination, imperfections, and/or roughness. (With the roughness being on a scale that is small compared to the size [diameter] of the drop.) In contrast to both the intrinsic and apparent contact angle, the dynamic contact angle θd is measured on a drop that is changing size or position and not necessarily on a horizontal surface. In this invention the term contact angle θ will be used as a general term encompassing whichever of these three contact angles that is applicable for the situation.
Because the wettability of liquids on solid surface is important to quantify, there have been many approaches used to measure the contact angle of a liquid on a solid surface. Prior art approaches have included the sessile drop method, the tilting plate method, the Wilhelmy plate, and the capillary rise method. Typically, the wettability of a surface is determined largely by the intrinsic contact angle θi that the liquid makes with the solid surface.
It should be noted that although the contact angle θ, is the most common way to measure and assesses the phenomenon of liquid-solid wetting, it alone does not adequately describe all aspects of solid-liquid interaction in every situation. For example, the measurement of the contact angle alone is not always precise in quantifying the area of contact between a liquid and a solid surface. That is, the concept of wettability in its most precise definition is based on the contact angle that the liquid makes with the surface at the perimeter of the liquid. It does not deal, for instance, with the area of contact between the liquid and the solid surface.
In some situations it is desirable to be able to alter the wettability of a surface. That is, to be able to increase or decrease the area of the surface in intimate contact with the liquid. In the past, this has only been possible by changing the character of the liquid or of the solid in some manner, such as, by employing a liquid additive (for example, a surfactant), applying a surface coating, or changing the surface energy, for example. For some liquid-solid systems it is not desirable to modify either the solid surface or the liquid.
In this invention it will be demonstrated that it is easily possible to modify the area of liquid-solid contact by altering only the non-planar features on a solid surface or the shape of a capillary. The liquid-solid contact angle in this situation will remain unchanged. Thus, according to the definition of wettability, the wettability has not changed even though a casual observer would describe this transition in liquid-solid contact as changing from apparent wetting to apparent non-wetting behavior (or vice verse). Wetting and non-wetting do not at all convey the same meaning as fully contacting and partially contacting behavior between a liquid and a solid. Thus, although they can be used interchangeably in many situations, the degree or area of solid-liquid contact is preferable to the degree of wetting when describing the phenomena that this patent addresses.
Previously, knowledge of the relationship between the contact angle and the degree of solid-liquid contact was limited to planar horizontal surfaces and cylindrical capillaries. The relationship between contact angle and the area of solid-liquid contact on non-planar and non-horizontal surfaces as well as in capillaries with varying axial dimensions, cross-sectional shapes, and axial shapes has not previously been quantified. An understanding of the contact angle θ acting in concert with localized non-planar surface features or specific capillary geometries, which is one distinctive feature of this invention, may be used to increase or decrease the area of contact between a liquid and a solid surface.
In the prior art, the degree of contact of a liquid with a surface is determined solely by the contact angle θ that the liquid makes with the solid surface. In the present invention, the area of contact of a liquid with a surface or a portion of a surface has also been found to be influenced by the magnitude of the included angle δ in degrees between opposing portions of the surface(s) of the material(s). These opposing surfaces can take numerous forms, such as, plates, pits, pores, trenches, capillaries, etc. The applicants have found that there is a transitional included angle φt for both wetting and non-wetting liquids at which wetting behavior and thus the area of contact between the liquid and the solid surface changes.
This type of surface modification has application in lubrication of sliding surfaces, fuel/catalyst interactions, adherence of coatings, heat transfer and any other solid-liquid combination with a desired wettability.